1. Field of the Invention
The invention relates to a process and a device for improving surfaces, in which the surfaces of objects are improved by applying a layer or the surface properties of the object are improved by modifying the regions close to the surface.
Layers which are very stable and therefore may be very thin, can be applied in most cases only using plasma-assisted processes, since the equilibrium temperatures for layer formation are very high (in most cases far above 800xc2x0 C.) and most objects are damaged at such temperatures.
The simultaneous plasma-assisted coating of large surfaces with layers of excellent quality has hitherto only been possible in high or fine vacuum. Examples are magnetron sputtering, plasma-activated vapor deposition, vacuum-arc coating and the HF-CVD process. However, high and fine vacuum processes have the disadvantage that the expense in terms of apparatus for vacuum production is high and in addition, the treatment time is extended by the times for evacuation and airing of the treatment chamber.
2. Description of the Related Art
Known processes which operate in rough vacuum or even at atmospheric pressure are plasma spraying and the RPJ process (radio frequency plasma jet, L. Bardos, et al., Surface and Coatings Technology 72 (1995), page 174-180). These processes do indeed achieve very high layer growth rates, but only on very small surfaces of a few mm2 to cm2. Indeed, these nozzle-like coating tools may be moved for coating larger surfaces by means of a robot, but in order to be able to deposit large quantities of material in a short time, an extremely high electrical power has to be supplied to the tool, which leads to considerable heat and material problems in the tool itself.
A further disadvantage of this process is that material transport is effected by an extremely intensive flow of gas at very high gas speed, resulting in high consumption of working gas.
In addition, the layers grown at considerable local layer growth rates in most cases have reduced quality, in the case of plasma spraying there is merely condensation of molten droplets which do not undergo particularly intimate bonding to one another and to the substrate and in addition form a tight network of pores. In the case of the RPJ process there is also the fact that the energy has to be supplied in the form of high frequency and is converted with very low efficiency, so that on the whole considerable costs for current supply result.
A process for treating substrate surfaces, in which plasma-activation is caused by a hollow cathode luminous discharge (German 19 505 268) and inert gas is passed through the hollow cathode to suppress coating of the cathode surface, is also known. However, this process has four crucial disadvantages. Firstly, a large amount of inert gas is consumed for effective reactive gas displacement from the hollow cathode. Secondly, very large vacuum pumps are required to pump the spent inert gas to the atmosphere. Thirdly, a large part of the plasma energy is lost, since the reactive gas does not pass into the zone of the greatest plasma intensity, and fourthly the process may only be carried out briefly using very low power and hence low layer growth rate, since cathode cooling is not provided and hence there is overheating and destruction of the cathode and damage to the substrate. Inasmuch as cooling is provided, there is the serious disadvantage that high expense in terms of construction is produced when reproducing the hollow cathodes due to the cooling system and in addition the size of the individual hollow cathode plus the distance to the neighboring hollow cathode must have a minimum value, resulting in the process remaining restricted to a pressure range below about 10 mbar, which causes vacuum costs accordingly.
The object of the invention is therefore to provide a process and a device for improving surfaces, which may be operated both in rough vacuum and at atmospheric pressure and by means of which improving of high quality is achieved with good efficiency and without or with only low inert gas consumption.
This object is achieved according to the invention by the features of the independent claims.
It has been shown, surprisingly, that the hollow cathode cleans itself at a sufficiently high cathode temperature, that is the parasitic deposits stemming from the reactive gas are desorbed, evaporated, atomized or dissolved or at least become electrically conductive. Hence, stable operation of the hollow cathode is possible even without considerable flow of inert gas.
Advantageous further developments and improvements are possible due to the measures given in the sub-claims.
So that the hollow cathode cleans itself, the reactive gas, to which inert gas may be admixed for dilution, may be passed wholly or partly through the hollow cathodes, resulting in the efficiency improving, since the reactive gas may be passed into the zone of greatest plasma intensity.
The cooled anode arranged at a slight distance from the hollow cathode facilitates cooling of the hollow cathode so that the temperature of the hollow cathode stabilizes at the self-cleaning temperature taking into account the heat removal by the reactive gas, that is may be kept at defined temperature conditions. In addition, a thermal resistance may be provided between anode and cathode.
The shape of the cathode may be matched to the shape of the objects to be treated, resulting in it being possible to move the hollow cathode close to the surface to be treated, resulting in it being possible also to work at higher pressure. This is further promoted in that the hollow cathode is formed from a plurality of hollow cathode elements arranged like a matrix.
By way of example to produce structures on the surface the latter may be treated locally or locally differently by for example the hollow cathode matrix being designed as a type of xe2x80x9cmaskxe2x80x9d or the shape of the hollow cathodes being matched according to the required structure.